1. Field of the Invention:
This invention relates to an apparatus for detecting radiation containing medical information from an object under examination in a two-dimensional plane and for producing a two-dimensional image of the object as the medical information.
2. Description of the Prior Art:
A scintillation camera is a practical available radiation imaging apparatus.
The Anger type scintillation camera as the most typical scintillation camera is known. See, for example U.S. Pat. No. 3,011,057 to Anger issued on Nov. 28 , 1961. The principle operation of the Anger type scintillation camera is as follows. A scintillator made of NaI(Tl) crystal is arranged to receive an amount of radiation which is administered to the object, e.g., a patient. Due to the scintillation phenomenon, the scintillator emits light (fluorescent emission) from the part (position) of the scintillator in which the radiation, e.g., .gamma.-rays (gamma-rays) are incident.
In general, several tens of photomultiplier tubes, for example, 30 to 90 two-inch photomultiplier tubes, are positioned to receive the scintillation light derived from the scintillator. These tubes are arranged in a two-dimensional plane. The position of the fluorescent emission in the scintillator is discriminated by proportionally calculating the amount of the detected light of the photomultiplier tubes. While these fluorescent emissions are counted by photon counters, the detection operation by the photomultiplier tubes continues until the counting results reach the predetermined values. As a result, two items of data are obtained for the scintillation emission position in the two-dimensional plane and for the intensity distribution of the .gamma.-rays. The two-dimensional image is displayed in accordance with those data, so that a medical diagnosis can be made by observing the distribution image of the radioisotope administered to the patient.
In accordance with the above-described conventional system, an image consisting of 256.times.256 pixels is constructed by processing the signal which is obtained from 64 pieces of the photomultiplier tubes. The system provides poor spatial resolution and requires correction to obtain a homogeneous quality of the image. Due to its inherent operation, i.e., the proportional calculation cannot be executed if more than two photons of .gamma.-rays are simultaneously incident on the entire reception field of the scintillation camera. Accordingly, the counting efficiency of the .gamma.-rays is extremely deteriorated, and the total detection time is increased, resulting in a lengthy diagnosis.
As is known in the art, the .gamma.-ray incident upon the scintillator usually contains the scattered rays which are unnecessary for measurement. These scattered rays have a lower energy level than primary rays. Accordingly, the conventional system eliminates the scattered rays by utilizing energy analysis, e.g., a pulse-height selector, or analyzer. However, the energy analyzing capability depends on the number of visual photons that contribute to the total detection currents of the photomultiplier tubes.
In the Anger type scintillation camera, there are the drawbacks of the energy conversion efficiency of the scintillator, the photon collection efficiency, conversion efficiency, and the energy resolution of the photomultiplier tube. Moreover, since the photomultiplier tube is employed, the total weight of the detector assembly, including the scintillator and the photomultiplier tubes, becomes too great. The heavy camera causes a more serious problem in emission computer tomography in which the camera assembly is required to be rotated around the object. In addition, since the measurement is carried out to calculate the detection currents that are derived from a plurality of photomultiplier tubes, the detection characteristics of the photomultiplier tubes must be accurately corrected, resulting in a high manufacturing cost and cumbersome maintenance.
It is therefore an object of the present invention to avoid the above conventional drawbacks, and to provide a radiation imaging apparatus with improved radiation counting characteristics and spatial and energy resolutions.
Another object of the present invention is to realize homogeneous image quality without any adjustment of the apparatus.
Still another object of the present invention is to provide a radiation imaging apparatus useful in, especially, the field of nuclear medical diagnosis, with which a compact and lower cost two-dimensional radiation detector can be made without requiring cumbersome maintenance.